(1) Field of the Invention
The present teachings relate to an underwater projectile for neutralizing undersea targets from a relatively long range. More particularly, the present teachings relate to the tail or end portion of a supercavitating projectile and to arrangements and methods for emitting gases from the end portion to stabilize the projectile and reduce viscous drag.
(2) Description of the Prior Art
Projectiles fired from underwater guns can effectively travel large distances by making use of supercavitation. Supercavitation occurs when a body, such as a projectile, travels through water at a relatively high-speed and a vaporous cavity begins to form at its tip. With proper projectile design, a vaporous cavity envelops the entire projectile.
In FIG. 1, a known supercavitating projectile 10 is shown in which a vaporous cavity 12 surrounds the projectile 10. The projectile 10 is shown with a flared afterbody 16 emitted from its tail portion. As the projectile 10 attains relatively high-speeds, the projectile does not contact water except at a cavitator tip 14 and during occasional collisions with the walls of the vaporous cavity 12, referred to as tail-slap. As a result of the formation of the vaporous cavity 12, a viscous drag on the projectile can be significantly reduced compared to a fully-wetted operation.
Tail-slap is relevant both to the stabilization of projectiles and to the minimization of drag. When traveling at relatively small angles of attack, supercavitating projectiles generally do not contact the vaporous cavity except at the tip of the projectile, as shown in FIG. 1. Forces produced by the tip are generally aligned with the major axis of the projectile and no significant yaw forces are produced. However, if the body of the projectile is perturbed and begins to yaw, substantially no restoring forces are experienced until the flared afterbody comes into contact with the cavity wall. When this occurs, a restoring force substantially proportional to the angle of the emitted flare can be produced. This restoring force can push the projectile back in the opposite direction and the projectile will then yaw in the other direction until the cavity wall on the opposite side is impacted. This rattling back and forth is the basic stabilization mechanism of non-finned projectiles. Every time the projectile impacts the cavity wall it experiences a drag force and a bending moment. If the bending moment is large enough, the projectile can break in flight.
Another related concern with the operation of projectiles is the issue of depth and speed with respect to the generation to form and the size of the cavity is a function of the speed of the projectile and the size of the cavitator tip. As the projectile begins to travel down-range, it begins to slow due to drag generated at the tip, resulting in the size of the cavity shrinking. The cavity continues to shrink as the projectile decelerates until the cavity can no longer envelop the entire projectile. The water pressure surrounding the projectile can also influence the operation of the projectile. The size of the cavity is inversely proportional to the ambient pressure. Consequently, projectiles are incapable of traveling the same distance at a greater depth compared to a shallower depth.
It is known that enlarging the cavitation bubble surrounding an underwater projectile reduces hydrodynamic drag. In Miskelly (U.S. Pat. No. 6,405,653) a projectile is disclosed that includes an internal ventilation system for venting propellant combustion gases to an exterior of the projectile near the front or nose portion thereof. The vented combustion gases emitted from the nose portion serve to expand the naturally occurring cavitation bubble formed as the projectile travels through the water with the result of reducing hydrodynamic drag. However, the Miskelly reference does not disclose a way of eliminating the occurrence of tail-slap during travel of the projectile.
As such, a need continues to exist for eliminating or reducing the occurrence of tail-slap in projectiles. There also exists a need to achieve improved accuracy and stability and to extend the range of projectiles.